Computerized tomography equipment, such as CT scanning apparatus, is widely used as a diagnostic tool for analyzing the internal profile of an object under study, such as for the medical diagnosis of a human body organ. Such equipment can provide a two or three-dimensional composite "picture" of the object by analyzing a plurality of radiation images taken of the object at different orientations. A usual source of imaging radiation is a source of X-rays located on one side of the object, with the images being developed by one or more X-ray detectors located on the opposite side of the object, whose signal outputs are converted into digital signals which are analyzed by computer.
The CT data acquisition system receives the output from the detectors, and, under digital control to correlate a particular signal with a particular orientation, conditions, amplifies and converts the detector output signals into useful digital data form suitable for subsequent analysis.
Typical data acquisition circuitry has four main components: a front-end signal conditioner, an analog signal multiplexer, a data converter, and a digital control. The front-end signal conditioner serves to convert relatively low level analog signals from the detectors into low output impedance signals for the rest of the acquisition system. Normally, each detector channel or line has at least some dedicated signal conditioning circuitry associated with it. The function of the multiplexer is to take the signals from the different detector channels and enable them to be processed (in time-sharing fashion) along common channels, thereby reducing the number of components needed in the follow-on circuitry. The analog output of the multiplexer is fed into a data converter to transform the analog signals into corresponding digital signal information appropriately converted to digital form. The whole process operates under the direction of the digital control circuitry.
The data acquisition circuitry of conventional high performance CT systems utilizes more than one data converter because of the combined sampling rate and accuracy requirements. The data converter is often comprised of two primary elements, a floating point amplifier and an analog-to-digital (A/D) converter. To ensure that the input to the A/D converter is always greater than some minimum value, the floating point amplifier operates to provide greater amplification for smaller magnitude input signals, with the amount of amplification given to a particular signal being selected as a function of the magnitude of the input signal.
One approach to previous data conversion circuitry (discussed further below in reference to FIG. 4) utilizes a programmable or selectable gain amplifier in which gain is set by changing the feedback path through switching the point of connection to a plurality of resistors connected in series between input and output terminals of the amplifier. Because this approach achieves different gains by varying the feedback resistance of the same amplifier, the settling times are long and different for each gain selection. Also, implementation of an offset drift correction (auto-zero) capability is complicated and cumbersome, as each gain configuration requires a different amount of offset or offset value. Furthermore, gain adjustment is complicated because the same resistors affect more than one gain selection.
A second approach (discussed later in reference to FIG. 5) utilizes a plurality of amplifiers in parallel, each configured for a different single gain setting, and means for selecting which one of the amplify paths will be used to amplifier a given signal. This approach is faster than the first approach but requires a different amplifier, with a corresponding different settling time, for each gain setting. Also, because separate amplifier configurations are used, each gain setting will require its own auto-zero setting circuitry for offset voltage correction.